From: Nasir Khan <rubylearner@...>
Date: 2007-03-04T07:22:01+09:00
Subject: Find where in hierarchy was the message handled

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If you have an hieararchy of classes and the parent class method has a
complex dispatch logic to the children then you may like to test if the
dispatch happens correctly or not.

Here is an example -
I have many types of audio players and there is a base player doing some
common stuff.

#--- file players.rb -------
class BasePlayer
  def play
    # do base players job
  end
end


class Mp3Player < BasePlayer
  def play
    # play mp3 files
  end
end
#--------------------------


I oftentimes in my test want to know if the base class's or derived class's
play method was invoked. This example may not be enough to convince you of
the requirement but I do have use cases in which I want to know where in
hierarchy was the method handled. So I have this test with associated
wrapper module.


#-----file tc_players.rb----------

require 'test/unit'
require 'players'

class PlayerTestCase < Test::Unit::TestCase

  def setup
    @b = BasePlayer.new
    @m = Mp3Player.new
  end

  def test_play
    @b.extend(PlayerExtender)
    @m.extend(PlayerExtender)
    assert_equal( "BasePlayer",  @b.play[0] )
    assert_equal( "Mp3Player",  @m.play[0] )
  end

end

module PlayerExtender

  def PlayerExtender.extended(xtendee)
    xtendee.class.instance_methods(false).each do |m|
      PlayerExtender.wrap_method(xtendee.class, m)
    end
  end

  def PlayerExtender.wrap_method(klass, meth)
    klass.class_eval do
      alias_method "old_#{meth}", "#{meth}"
      define_method(meth) do |*args|
        [self.class.name, self.send("old_#{meth}",*args)]
      end
    end
  end
end

#-------------------------

I am posting this here for two reasons.

(a) Please let me know if this can be improved or I have overlooked a
simpler solution
(b) In the hope that this could be useful to others.


Thanks

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